A plurality of battery cells employing secondary batteries is conventionally connected in series to form an assembled battery for the purpose of improving handling performance and simplifying the structure. If in such an assembled battery, a lithium-ion battery is used for each battery cell, a method is known in which each battery cell is connected to a voltage detection line to measure the voltage, and the state of each battery cell is detected based on the measurement result to manage the assembled battery.
In the above assembled battery management method, if connection failure occurs between the battery cell and a voltage measurement circuit due to a break in the voltage detection line or the like, the voltage across the battery cell cannot be correctly measured. Therefore, the assembled battery cannot be appropriately managed. Hence, a method for detecting connection failure as in the following PTL 1 is known as means for solving this. In this method, a short circuit is caused via a resistor between the voltage detection lines connected respectively to the positive and negative electrodes of the battery cell, for a predetermined time. The short circuit is subsequently canceled to detect whether the voltage between the voltage detection lines is approximately a short circuit voltage. Accordingly, whether or not connection failure has occurred is judged.